Can bilateral pes planus (flat feet) cause knee pain?

Can Bilateral Pes Planus Cause Knee Pain?

Yes, bilateral pes planus (flat feet) can cause knee pain, particularly in the medial (inner) compartment of the knee, and this association is supported by both biomechanical evidence and clinical outcomes data.

The Evidence for the Association

The relationship between bilateral flat feet and knee pain is well-established through multiple research studies. Bilateral flat feet are significantly associated with increased knee pain, with patients showing 5.48 times higher odds of experiencing knee pain compared to those without flat feet 1. This association is specific to bilateral involvement—unilateral flat feet do not show the same significant relationship with knee pain 1.

Severity Matters

The severity of pes planus directly correlates with knee symptoms:

• Mild pes planus: No increased risk of knee pain compared to normal arches 2
• Moderate to severe pes planus: Nearly double the rate of anterior knee pain (7% vs 4% in controls) 2
• The more severe the flat foot deformity (measured by higher Meary's angle and foot arch index), the worse the knee function and greater the pain intensity 3

Mechanism: Altered Biomechanics and Cartilage Damage

Pes planus doesn't just cause pain—it's associated with structural knee damage. Planus foot morphology increases the odds of medial tibiofemoral cartilage damage by 1.4 times, with this risk increasing linearly as the foot becomes flatter 4. The mechanism involves:

• Altered lower limb alignment during gait
• Increased medial compartment loading (70-80% of joint load passes through the medial compartment during mid-stance) 5
• Changes in the ground reaction force distribution
• Increased knee valgus and altered pelvic mechanics during walking 6

Age and Population Considerations

The association between flat feet and knee pain spans different age groups:

• Adolescents: Moderate and severe pes planus shows doubled rates of anterior knee pain 2
• Older adults: Flat foot morphology is associated with frequent knee pain and medial tibiofemoral cartilage damage 4
• Middle-aged adults with knee OA: Those with bilateral flat feet report significantly higher knee pain scores (11.3 vs 6.58 points) 1

Clinical Implications and Management

When to Suspect Pes Planus as a Contributor

Evaluate for pes planus when patients present with:

• Medial knee pain without clear traumatic etiology
• Bilateral knee symptoms
• Pain worsened with prolonged standing or walking
• History of "flat feet" or foot pronation

Assessment Approach

Physical examination should include:

• Foot arch measurement: Use the Staheli Arch Index or foot arch index 1, 4
• Meary's angle assessment: Radiographic measurement on lateral foot films (normal <4°, mild 4-15°, moderate 15-30°, severe >30°) 3
• Knee alignment evaluation: Assess for valgus deformity and medial compartment tenderness
• Gait observation: Look for excessive foot pronation and altered knee mechanics 6

Treatment Strategy

Orthotic intervention is the primary evidence-based treatment for addressing the biomechanical link between pes planus and knee pain:

1. Shoe orthotics to correct overpronation or pes planus are recommended for anatomic misalignment 7

2. Laterally wedged insoles are recommended by major guidelines (Osteoarthritis Research Society International and American College of Rheumatology) for medial compartment knee OA associated with foot pronation 5. However, note that:

   • Full-length wedged insoles show more consistent biomechanical effects than heel wedges 5
   • Response varies between individuals—some patients show no benefit or even increased medial loading 5
   • Consider trial use with assessment of response

3. Prefabricated foot orthoses should be prescribed to those who respond favorably to treatment direction tests (e.g., observing pain reduction during squatting with vs. without orthoses) 8, 9

4. Appropriate footwear: Stable, supportive shoes (worn at least 6 hours daily) show benefit for knee OA pain 10

Important Caveats

• The evidence for orthotic effectiveness is mixed, with some studies showing no consistent pain relief despite biomechanical changes 5
• Early intervention and orthotic management may be beneficial in mitigating progression of knee OA in patients with pes planus 3
• Treatment should address both the foot mechanics and knee-specific impairments (strengthening, exercise therapy) for optimal outcomes
• Not all patients with flat feet develop knee pain—other factors (obesity, muscle weakness, age) contribute to the clinical picture 1, 3

The Bottom Line for Clinical Practice

When evaluating knee pain, particularly medial compartment pain, always assess foot posture bilaterally. If moderate to severe bilateral pes planus is present, it represents a modifiable biomechanical factor that should be addressed through orthotic management alongside standard knee-directed treatments. The goal is to reduce abnormal medial compartment loading and potentially slow cartilage damage progression.